Instructions
Let Ostrava Die
Approach Ostrava in the Boletarian Palace, Phalanx Archstone or Tower Knight Archstone. He's surrounded by enemies, who will initiate an attack on him as you approach.
Wait while the enemies kill him. Once he's dead, move in and finish them off.
Loot Ostrava's corpse. You'll find the Mausoleum Key on him. If you enter a new map without taking the key, it will disappear forever.
Save Ostrava
Approach Ostrava in the Boletarian Palace, Phalanx Archstone and Tower Knight Archstone. So long as you saved him in the previous level, he'll appear. Each time, he's surrounded by enemies, and, as you approach, they'll initiate an attack on him.
Kill the enemies. Afterwards, speak with Ostrava. He'll give you a reward. In the Palace, he gives you a Brass Telescope. In the Phalanx Archstone, he gives you three Dark Moon Grasses. In the Tower Knight Archstone, he gives you a Pure Clearstone.
Speak with him in the Penetrator Archstone. He'll give you the Mausoleum Key, as well as all his other equipment.